Sorry, please forgive delay. I had to take a few days to fight my own beast.
You brought up some very interesting points; and yes, I agree, it's very possible that these monsters can stay hidden then, all of a sudden, one day: surprise...you're dead.
What you say is also true: the L3 larvae can "encyst" for even longer than 2 years, it seems.
A few points though:
First: large and small strongyles are very different species....and only the large ones seem to disseminate (hide/scatter); small strongyles are the ones known to cause problems because of encystment in gut wall.)
2) everything I have read says that strongyles MUST leave your body in order to complete their life cycle. So when you talk about adults laying eggs....that's ok (kinda :/) because those eggs LEAVE your body right away, via feces. Done. Gone. Flush toilet. Wash hands.
The larva can't live inside of a host until they reach L3 stage. They must live in a pile of sh*t, literally, until they have matured to L3; this takes a variable amount of time, depending on external temps. Then, L3 larvae just hang out on blades of grass, waiting to be eaten by a passing herbivore, which is how they get into a warm-blooded animal's GI tract. So unless you're grazing in a horse pasture, you're Unlikely to reinfect yourself. Additionally, any egg-laying adults inside of you are pretty quickly dispatched with our tried-and-true anthelmintics.
3) Ivermectin is "systemic" meaning it is distributed throughout your body, via blood. So it can leave the GI tract to get the disseminated worms(which are the large strongyles, primarily...they go to the liver or pancreas, then return to the gut to lay eggs.) Iver kills large strongyles and small ones (supposedly, but resistance is growing).
4) It appears that moxidextin is still pretty effective in treating the encysted stages.
So what I'm saying ...the important part: Those strongyles are NOT staying in your body for their entire lives. They leave as eggs, shortly after being "laid"....they CANNOT hang out in your gut through their 3 larval stages....so if you are getting L3's embedded in your intestine, you are picking them up from your environment somehow.
I really hope this helps...It took me a while